Portal Bracing

[txeng91]

Has anyone used bracing similar to what’s shown in the attached image? They call it unsymmetrical portal bracing. I am looking at using this configuration to work around a door. I’m a little uncomfortable having that pinned joint where all of the braces meet under compression and subject to buckling. My thought is to run the primary brace from the base of the column up to the beam above and have the other 2 braces that branch off to the beam-column joints above attached to the primary brace with gusset plates. I typically use more traditional bracing methods so any insight on this or something that might be more efficient would be greatly appreciated. The bay is 10’ wide by 14 ft tall with a 4’x9’ opening at about third span. I originally had a diagonal brace but the door heights were raised from 7’ to 9’ and the brace will no longer clear the door.

 

[wannabeSE]

Never seen such a thing. Is a forth brace out-of-plane required for stability?

 

[KootK]

Yeah, the demand for WSE's fourth brace would seem to be the issue. And it's hard to imagine that you'd get such a brace. Here are some wacky alternatives. If you're high seismic, it's a different conversation of course.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[txeng91]

Yes those were my thoughts as well. That first one KootK drew up is what I was thinking of doing. Looks like it’s ny best option, I don’t really have enough room on the opposite side of the door to Run a brace on that side.

 

[RPMG]

Scrap it. Use moment connections

 

[Guest262653]

I agree that for compression forces that central node just buckles sideways but this bracing system may carry tension forces if loaded from left to right (in your sketch). If you have other systems in your building that may carry horizontal loads coming from the other side, thys system might be feasible, albeit not as stiff as a typical diagonal bracing.
I've seen these systems in an excellent french rehabilitation book ("Guide de la rehabilitation avec l'acier" - Link). Here are some photos I took of a few pages which show this system applied in the context of rehabilitation:

 

[jayrod12]

I see this type of bracing used for crane systems quite often as well. But it's usually a double sided entity like shown on the last attachment by avscorreia

 

[txeng91]

Yeah I kind of figured it might be a tension only system. It’s the only bay bracing the structure so I need it to work in compression too. Since the bay is only 12’ wide I’m just going to run a flat wide flange between columns at the node to brace it out of plane. Thanks for the responses.

 

[BAretired]

Wouldn't it be simpler to use KootK's first wacky alternative?

See attached:

BA

 

[dik]

There's no need to extend the sloped vertical member to the beam... just complicates things.

Dik

 

[KootK]

There's no need to extend the sloped vertical member to the beam... just complicates things.

It does complicate things but there is a need. The need is to supply out of plane stability to a C-C-C joint. There are other ways but that was the intent of that option.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.